Vehicle Crashworthiness and Occupant Protection - Chapter 3

Vehicle Crashworthiness and Occupant Protection
the bushing using contacts between surfaces that simulate the presence of the
incompressible part of the rubber.
The powertrain model contains models for the engine, transmission, driveshaft
and engine mounts, including their rubber bushings. The goal of this submodel is
the correct simulation of the engine and driveshaft kinematics in order to account
for the possible contacts with structural parts later on in the crash event. To
achieve correct kinematics, it is necessary to accurately model the mass, rotational
inertia and center of gravity position of the engine and transmission block.
Furthermore, the engine mounts must be modeled in a similar way to the subframe
mounts. This has proven to be the decisive factor in determining the relative
rotation between powertrain and body-in-white. A cylindrical shell model with 5
or 6 elements over the circumference is typically used to model driveshafts in
order to simulate potential contacts with brackets in the car body. Spherical joints
connect the driveshafts to the wheel knuckles and to the transmission block. In
order to avoid small timesteps, rigid body definitions are usually superimposed
on the powertrain subsystem models. In order to obtain smooth contact forces
between engine block and structure, the external geometry of the engine block
must be accurately modeled using elements of a size not much larger then the
ones used for the car body.
All the main parts of the front axle are also modeled using shell elements starting
from CAD surface or line data. For a typical McPherson front axle, this subsystem
will contain detailed modeling of the wheel knuckle, suspension strut, lower control
arm and stabilizer bar. The same remarks made in the description of the powertrain
system remain valid here. A detailed shell model is necessary to ensure that all
potential contacts with structural parts can occur in the model. This means that
the stabilizer bar is modeled as a cylindrical bar with 5 or 6 shells over the
circumference. If these elements have a very small dimension, mass scaling or
other techniques can be used in order to prevent a dramatic decrease in the
calculated stable timestep. Usually the rubber bushings in the chassis model are
not modeled in detail, as is done for the subframe and engine mounts. Rather, a
series of revolute, cylindrical and spherical joints provide the correct hinges in
the connections between the chassis parts as well as between chassis and car
body structure. Obviously, a similar strategy is employed to model the rear axle
parts in the case of rear impact simulations.
The wheels are connected to the axle models. The wheels consist of detailed
geometrical models for wheel rim, brake disk and outer tire. The wheel rim and the
brake disk are usually rigidly connected to the wheel rim, thus preventing the
wheel from rotating. This must obviously be improved if mishandling simulations
are performed, but is acceptable for crash simulations. A detailed model is
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